A number of power tools have been produced to facilitate forming a work piece into a desired shape. One such power tool is a miter saw. Miter saws present a safety concern because the saw blade of the miter saw is typically very sharp and moving at a high rate of speed. Accordingly, severe injury such as severed digits and deep lacerations can occur almost instantaneously. A number of different safety systems have been developed for miter saws in response to the dangers inherent in an exposed blade moving at high speed. One such safety system is a blade guard. Blade guards movably enclose the saw blade, thereby providing a physical barrier that must be moved before the rotating blade is exposed. While blade guards are effective to prevent some injuries, a user's finger is nonetheless in proximity to the moving blade, particularly when attempting to secure a work piece as the miter saw is used to shape the work piece.
Miter saw safety systems have been developed which are intended to stop the blade when a user's hand approaches or touches the blade. Various stopping devices have been developed including braking devices which are physically inserted into the teeth of the blade. In general, upon detection of a person in the vicinity of the blade, a signal is processed and sent to a reaction system such as a brake mechanism to stop blade rotation within a short period of time. One such system is disclosed in U.S. Patent Publication No. 2011/0048197. In other systems, reaction systems incorporate a mechanical or electrical brake. Pyrotechnic firing is also used in some reaction systems to force a moving blade or other shaping device, initially extending through a slot or hole in a table, underneath the table.
In all of these systems, the ability to place the system into a safe condition is predicated upon timely sensing that an unsafe condition exists. The sensitivity of a sensing system must therefore be sufficient to minimize risk of injury. If the system is too sensitive, however, then the tool will be placed into safe condition when there is actually no danger to a user. Such inadvertent activations of the injury mitigation system can be very inconvenient and, in some systems, costly.
What is needed therefore is a simple and reliable configuration which reduces the potential for inadvertent activations of an injury mitigation system while providing increased sensitivity to unsafe conditions.